Linear junction laser diodes are used in many applications in which the emitted rays must be collimated and projected by appropriate optics to illuminate a distant spot or object. Such applications include weapon rangefinders, illuminators, optical security systems, laser spot projectors, page scanners, and copiers.
A laser diode is a commercially available type of semiconductor laser. A typical off-the shelf pulsed laser diode is the Stantel LE Series type LE25-02. This diode is of GaAs/GaAlAs single heterostructure semiconductor laser design. This and similarly constructed laser diodes emit uncollimated rays over a wide angle of divergence from a high aspect ratio linear junction or emitting region. The LE25-02 diode has a divergence of 28 degrees perpendicular to the emitting junction and 20 degrees parallel to the emitting junction. A collimating lens of practical size for most applications with long junctions can therefore collect only a relatively small percentage of the emitted power.
Prior collimation optics for diodes simply use combinations of lenses (spherical and aspheric including cylindrical) which are placed far in front of the diode. Typical spacing of prior art lenses from the diode range from about 30 to 1000 times junction length.
Collimating lens f/numbers are usually high (about 4 to 6) for high aspect ratio junctions. For low aspect ratio junctions (aspect ratio less than about 10), lower f/number optics can be used because the lens focal length can be much shorter. Long junctions require long focal lengths for low divergence and good collimation. Since lens diameter greatly affects optics weight, lens diameter is limited by weight. The f/number tends to increase as lens focal length increases. But high f/number optics do not collect diode power efficiently. For example, manufacturer,s data for the LE25-02 diode indicates that an f/4 lens will capture 36% of radiant power emitted by this diode at 25 degrees C. The remaining 63% misses the lens and is lost.
A further difficulty with collimating rays from long junctions is that simple optics project a collimated beam which has much greater divergence in one direction than the other (corresponding to the aspect ratio of the junction). This divergence difference must be accepted or cylindrical optics must be employed with two focal lengths--one focal length for collimating the junction width, another for collimating the junction length. Cylindrical collimation lenses have been found to be difficult to align and they are costly compared to spherical lenses.
Consequently, for long junction laser diodes, a need exists for a collimating means to improve power transfer from diode to collimation lenses (collection efficiency) which results in a projected beam from collimation lenses with reasonably equal divergences without the need for cylindrical collimation lenses. Preferably, such a collimating means will be optically simple, light weight, inexpensive, and will require little space within the device in which it is used.